Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Code division multiple access techniques transmit multiple information signals on the same channel, and differentiate each user sub-channel by encoding the channel with a unique spreading code. Many communication systems utilizing code division multiple access techniques comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system substantially eliminates co-channel interference and improves the bit energy-to-noise density ratio, Eb/No, on the forward link from a base station or cell site to a mobile receiver unit by modulating the information signals with Walsh orthogonal function sequences having a length equal to a power of 2 (2n). A more detailed discussion of the IS-95 standard is provided in “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” Telecommunication Industry Association Doc. No. TIA/EIA/IS-95(1993), incorporated by reference herein.
Walsh orthogonal function sequences having a length equal to a power of 2 exhibit excellent cross correlation properties that optimize the detection of the transmitted information (each codeword goes to zero unless modulated by itself). The constraint on the code length, however, directly limits the number of channels that can be supported in a given bandwidth. While “power of 2” Walsh codes are suitable for many applications that can match the number of channels to the available bandwidth, the efficiency and utility of CDMA networks could be significantly extended if further variation on the number of channels could be achieved.
Specifically, since each Walsh code must have a length that is equal to a power of 2, only a limited number of channel and bandwidth combinations are supported efficiently. For example, if a broadcasting system transmits 72 channels of audio information using current code division multiple access technology, then Walsh codes having a length of 128 must be utilized. Assuming each channel includes 96 Kilobits-per-second (Kbps) of audio information, the required system bandwidth is 12.29 Megahertz (MHz) (96 Kbps times 128 channels) and the bandwidth overhead is 5.38 Megahertz (MHz) (96 KBps times (128-72) channels). Thus, a need exists for a CDMA transmission system that does not require orthogonal spreading codes having a length that is a power-of 2. A further need exists for a CDMA transmission system that accommodates more channels in a given bandwidth.